1. Field of the Invention
The present invention relates to solid-state laser systems, and particularly to pulse-type solid-state laser systems that output high-power laser beams with relatively long pulse widths.
2. Description of the Related Art
A conventional solid-state laser system includes a solid-state laser medium, a light source for pumping the solid-state laser medium, and a laser resonator constituted from at least two reflecting mirrors arranged so as to flank the solid-state laser medium. The solid-state laser medium is pumped by the light source, thereby generating gain; a ray shuttling between the reflecting mirrors is amplified by means of the gain of the laser medium; and rays that are transmitted through a partial-reflection mirror, out of the reflecting mirrors, having partial-reflection characteristics in which part of rays are reflected and the rest are transmitted, are extracted as an output. The constitution in which planar mirrors are utilized as reflecting mirrors and a solid-state laser medium is situated at the center of the reflecting mirrors is a fundamental unit in cases where a solid-state laser system is made to output high power, without additional methods of adjusting a laser beam, by connecting the solid-state laser media in series, and the constitution is one of the most fundamental and useful ones.
In general, in a solid-state laser system, a solid-state laser medium generates heat after being pumped, thereby producing a temperature distribution; and whereupon a thermal-lens effect occurs in which the solid-state laser medium functions as a lens. The thermal-lens effect is enhanced approximately in proportion to a pumping input; the thermal-lens effect, enhanced in response to the pumping input, of the solid-state laser medium could produce a phenomenon in which the operation of the resonator becomes out of the conditions for stable oscillation, thereby ceasing the oscillation. The upper limit, of the strength of the thermal-lens effect, under which the resonator stably operates, is related to the length of the resonator; the shorter the length of the resonator is, the more stably it operates even with a strong thermal-lens effect. In other words, the resonator stably operates up to a high pumping input and a high output. Therefore, in order to obtain a high output, it is necessary to utilize a short-length resonator and to pump it strongly.
Meanwhile, in the case of a pulse-type laser system that performs pulse oscillation by means of an optical switch, there is a relationship that the shorter the length of the resonator is and the stronger the pumping is, the shorter the pulse width is.
As described above, in general, the high output and the relatively long pulse width in pulse oscillation require contradictory operating conditions.
With regard to a pulse-type solid-state laser system that generates a long-pulse-width laser beam by using a long length resonator, a resonator structure for enhancing the stability of the resonator has been proposed (e.g., refer to Patent reference 1). The resonator includes a solid-state laser medium; a first telescope for magnifying a laser ray entering from the one end thereof, i.e., a laser ray coming from the solid-state laser medium and for contracting the laser ray that has been reflected by a first planar reflecting mirror and enters thereto from the other end thereof, the first planar reflecting mirror for reflecting the laser ray that comes from the first telescope; a second telescope for magnifying the laser ray that has been reflected by the first planar reflecting mirror, contracted by the first telescope, amplified by the laser medium, and then enters thereto from the one end and for contracting the laser ray that enters thereto from the other end thereof, and a second planar reflecting mirror for reflecting the laser ray that enters thereto by way of the second telescope. The foregoing telescope is generally constituted from two lenses. In the laser system such as this, by utilizing the foregoing telescope and by making a laser beam that has been generated in the long-length resonator and that has been contracted to an appropriate size pass through the solid-state laser medium, the stable operation up to a high output has been implemented utilizing the long-length resonator. This constitution, because of using a long-length resonator, provides oscillation with relatively long pulse width.
[Patent reference 1] Japanese Laid-Open Patent Publication 2001-274491 (p. 6-7, FIG. 1)